Industrial fluid waste usually requires treatment before it can be safely discharged into the environment. Industrial fluid wastes often contain high amounts of contaminants such as organic compounds and heavy metallic species, and these contaminants need to be removed (or significantly reduced) before the waste is safe for disposal.
For example, acid mine (or metalliferous) drainage (AMD) is an industrial fluid waste that causes significant problems in the mining industry. AMD occurs when sulfide minerals in rocks are exposed to oxidizing conditions, for example, in coal and metal mining, highway construction or other large-scale excavations. There are many types of sulfide minerals, but iron sulfides (common in coal regions), pyrite and marcasite (FeS2) are the predominant AMD producers. Upon exposure to water and oxygen, pyritic minerals oxidize to form acidic, iron and sulfate-rich water.
Existing techniques for treating AMD include exposing the AMD to basic agents such as lime, which raises the pH of the AMD and causes many metallic species to precipitate. The precipitate is then allowed to settle and the treated water decanted. Other techniques, such as that described in U.S. Pat. No. 6,485,696, use ozone to rapidly oxidise specific metallic elements present in AMD. Ozone is bubbled through the AMD, which oxidises the metallic elements and causes them to precipitate. This technique may also involve the step of adding a basic agent to the ozone-treated water to cause other metallic elements to precipitate.